The present invention relates to graphic display systems generally, and particularly to digital circuits for controlling multi-bit color data when displaying color images in a raster scan CRT (cathode ray tube) display system.
In a typical monochromatic CRT display system, data bits stored in memory are mapped to the CRT screen, one bit per pixel or image element of the screen. The bit may be zero (off) representing black, or one (on) representing white.
In a typical color system, multiple bits rather than a single bit may be mapped to each pixel, each multiple representing a selected (predetermined) color value.
In such systems, two color states (e.g., a first color state and a second color state) may be specified for each pixel. Generally, however, such two-color systems do not provide for overlaying images in an efficient manner, i.e., for changing the color of selected pixels of an image and displaying (superimposing) the changed image over a previously unchanged version of the image, without requiring the system to perform additional time-consuming operations.
In one color system (see, for example, U.S. Pat. No. 4,888,582 issued to Schnarel, and incorporated in part herein), single bits are used in conjunction with multiple bits to specify color changes. The single bits are used as pixel control bits and the multiple bits as pixel color values. A single bit state of one indicates that a change is to be made in the color value of a pixel, whereas a single bit state of zero indicates that no change is to be made. Unlike two-color systems with first and second color features and no overlay feature, this system may be best described as a one-color system with overlay, the overlay being provided at the expense of the second color feature.
What is needed and would be useful therefore is an efficient two-color change system with overlay capability.